In recent years, a technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. Transistors have been applied to a wide range of electronic devices such as ICs and electro-optical devices, and transistors that are used as switching elements in image display devices are, in particular, urgently developed.
Metal oxides have been known as materials having semiconductor characteristics. Examples of the metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Document 1 and Patent Document 2).
Furthermore, the transistors including oxide semiconductors have a high field-effect mobility. Thus, with use of the transistors, a driver circuit in a display device or the like can be formed.